Holding the Small Stuff
You have to hold it to make it
By Robert B. Aronson
One commonly heard comment from machinists is that more than 80% of the parts manufactured in this country are palm size or smaller. And led by the medical and electronic industries, the trend is to ever-smaller products. One of the basic problems with small-part manufacture is how to hang onto parts during the manufacturing process.
Here’s a look at how several manufacturers have provided an answer.
The smallest chuck made by Kitagawa for NorthTech Workholding is a 4" (102-mm) PU pull back chuck that can hold parts as small as 10 mm in diam. "This chuck has 0.0004" [0.01-mm] repeatability," explains Bill Graham. "In all small part applications, part deformation is a major concern, particularly when working with delicate parts with tight tolerances." NorthTech’s solution is often the Kitagawa PU series pull back chuck with custom engineered top tooling. Gripping small workpieces with the PU is done with jaws that move both radially and along the Z axis to grip the part. This allows for length control and a grip that is full contact around the part with more pressure points.
If workpiece contamination is a concern, as it is especially in medical applications, it’s important to use a sealed chuck so lubricants inside the chuck don’t get on the workpiece. The PU chuck from NorthTech is a sealed chuck designed for this type of work, and has special seals that protect the part from contamination during machining.
The primary product of Paws Workholding (Clio, MI), is the multi-vise system which is available in three sizes, 8, 16, or 32 vises. The 2" (50-mm) wide vises are hydraulically activated with a flip of one handle or through the machining center’s controls. Each vise is activated by an embedded hydraulic cylinder and piston in the system’s baseplate, and is capable of up to 3000 lb (13,334 N) of force. The jaws are dowel pinned for repeatability and interchangeable.
The Paws system can also utilize interchangeable fixture plates to change from 8, 16, or 32 vises to V-blocks, expansion collets, or custom workholding components. Interchangeable fixture plates are made possible with a hydraulic baseplate having hydraulic cylinders every 2".
Most machine tool makers want to cover a range of capabilities from small to large," says Walter Schnecker, president, Datron Dynamics (Milford, NH). "But our machines are designed to work exclusively on small parts produced with small tooling. The technology that we’ve pioneered for efficient use of these small tools has created an interesting byproduct—the ability to utilize vacuum tables for workholding. The high-speed spindles are designed to achieve higher feeds, better cuts, and superior surfaces—all without unnecessary tool breakage—also facilitate new vacuum holding technology. This is because our high-speed spindles yield less force, and a good vacuum system can effectively hold parts in place during machining."
With small flat parts, Datron has had good success with a mounting system consisting of a vacuum plate fitted with a semi-permeable membrane. The part is positioned on the membrane and the vacuum drawn. The machining operation can cut through the membrane where necessary without harm to the machine tool. The membrane is discarded after each use.
"Vacuum works well if you have a flat edge, but if you have an irregular shape, there may be problems," explains Schnecker. For prismatic workpieces, a common holding technique at Datron is to create a holding fixture specifically for that part—a kind of reverse-image cassette that can secure the part. Then a conventional holding device will retain the part.
Another technique is to make the part with a web of tabs, then connect the part to a frame. After processing, the tabs are machined off.
"Low force is critical. You can’t use high torque. Small tools can operate at high rpm and have light cutting forces. Most customers are familiar with CNC and they like power. It’s analogous to using a driver club in golf. But with small parts you need a putter."
At Logansport Matsumoto Co. (LMC) (Logansport, IN), two of their products are keyed to small workpiece holding, their face-driving products and a 2.5" (63.5-mm) hydraulic chuck. The three-jaw can take non-round parts that can hold the part directly without a collet. It can run at 10,000 rpm and has 9.8 kN gripping force.
The face driver holds a part between an idler and a driving head. It can be used for hard turning of shafts as small as 2 mm in diam. These operations are done at over 10,000 rpm. With this unit, the entire workpiece OD can be machined in one operation.
Alpha Workholding Solutions (Hartland, WI), a company that specializes in magnetic workholding, uses a sandwich-like configuration for smaller parts. The part to be worked on is placed between a magnetic chuck and top tooling made of steel. Locating pins position the part.
Each part requires a holding piece configured to the workpiece. The holders use electro-permanent magnets. These magnets have the benefits of generating more holding power, maintaining some clamping force if you lose power, and not inducing heat. The workpiece may be just about any material and is not limited to those that can be magnetically attracted.
Small parts require special attention when using magnetic holding. There is less holding power acting on the part so cutting forces can skew it. The problem can be eliminated by applying the magnet to the sides of the part and "boxing in" an area of the part with fittings.
Haas Automation (Oxnard, CA) has focused on the machine, not the workholding. Most recently for the small-part market they offered the Office Mill (OM) and the Office Lathe (OL). The OM is available in four versions: OM-1, OM-1A, OM-2, and OM-2A. Configurations and travels are:
- OM-1: 8 × 8 × 8" (203 × 203 × 203-mm) travels with a 50,000-rpm NSK electric micromotor spindle.
- OM-1A : 8 × 8 × 12" (203 × 203 × 305-mm) travels with a Haas-built 30,000-rpm ISO 20-taper spindle and 20-pocket ATC.
- OM-2: 8 × 10 × 12" (203 × 254 × 305 mm) travels with a 50,000-rpm NSK electric micromotor spindle
- OM-2A: 8 × 10 × 12" travels with a Haas-built 30,000-rpm ISO 20-taper spindle and 20-pocket ATC.
For the OL-1 Office Lathe, the X-Z travels are 12 × 8" (305 × 203 mm), with an 11" (279-mm) swing over the cross slide and 4.95" (126-mm) swing over the table. An automatic air-collet closer is standard.
Operations of many machine tools can often be altered to match the parts. For example, Mazak (Florence, KY) engineers suggest using a collet chuck to hold the part. This gives more working room and allows the spindle to get closer to the work.
Vibration that might be ignored working with larger pieces can seriously degrade accuracy on a smaller piece so it’s important to ensure tool balance. Good balance is essential because vibration is more of a problem when working with small parts.
Master Work-Holding Inc., a subsidiary of Cogsdill Tool Products Inc., has specialized in the design and manufacture of custom workholding solutions using conventional hydraulic tombstone-type fixturing. They are now introducing a new process called Light Activated Adhesive Gripping (LAAG). This process utilizes a specially formulated adhesive for holding parts, activated by UV light. The parts are secured with a holding power of up to 6000 psi (43.4 MPa). In this process:
The process can be applied to lower-volume production without automation such as job shop applications. For more information, visit: www.masterworkholding.com. (see Manufacturing Engineering November 2004, p.37 for a detailed description of this technology.)
Jergens Inc. (Cleveland) offers an established line of workholding elements such as the Ball-Lok, which is made in a variety of sizes. Among the newer offerings is the Spinner Grip Flange Lock Nut.
When assembling a fixture, the nut is initially free spinning and then is tightened with a wrench. As it’s torqued, a cone within the nut flattens and locks the nut in place. The design is said to work especially well in high-vibration machining situations. When loosened, the cone returns to its original shape so the nut is reusable.
Small Part Manufacturing (Portland, OR) supplies both collets and fixtures. The smallest part they can handle is 0.060" (1.5 mm) using a Miyano turning center. One example is a stent insertion tool made in 20 min with a tolerance of ±0.002" (0.05 mm).
- Ceramic pins are fitted into a fixture plate.
- A meter-monitored syringe applies an adhesive to the tops of the pins.
- The workpiece is loaded onto the fixture and UV light cures the adhesive.
- The part is machined.
- The adhesive bond is broken mechanically.
- The fixture and finished part are cleaned to remove cured adhesive.
"With small parts, finish is often a problem. To provide protection, we use special collets with plastic bushings over the part," explains Stuart VanRiette. "The arrangement both reinforces the part and keeps it from being scored."
Kurt Manufacturing (Minneapolis) offers a series of vises for smaller parts. According to Michael Granlund, "The rules are the same as for large parts. You have to be concerned with finish, thin sections, production required, the part’s material, and whether the operation is automated or manual."
Kurt offers a new line of HD Vises that were designed to hold parts typically from 0.5" (13-mm) wide to the maximum carvable area of the vise. A typical application will hold two smaller parts per opening. In a typical HD Vise application this would be a total of four parts. With special jaws multiple parts can be held. In most cases Kurt’s HD line of Vises are intended for higher volume irregular shaped parts in which special carved jaws are used. The unique advantage of the Kurt HD Series Vise is the option to use the vise as a carvable vise or as a standard Double Lock Vise with hard jaw plates. The end user also has the option of converting the HD Vise to a hydraulic vise by replacing the internal nut and screw assembly should the need arise. With the HDM6 Vise the footprint is only 6 × 13.75" (152 × 349 mm), which allows for multiple vises per table.
Kosmek (Bridgeview, IL) offers hydraulic actuators including: link clamps, swing clamps, push/pull cylinders, and valves. These include the new Datum series, which features location and clamp cylinders for highly accurate location. Up to 3µm location accuracy can be achieved when loading a workpiece onto a fixture or positioning and clamping the entire fixture in the machine.
The smallest part Kosmek’s components can clamp depends on a number of factors, but a recently offered "hole clamp" positions and clamps an ID as small as 11 mm while allowing access to five sides of the part for machining.
Hydraulic clamping is used with small parts, because it is possible to achieve a very high clamping force from a compact component. With hydraulic clamping the fluid itself creates a cushion between the machining forces and the actuator.
One way to handle thin parts is to clamp the edges against hard stops, then support the center with work supports. These supports have a touch force as small as 0.4 lb (0.2 kg). This will eliminate any chatter and does not exert any excess force to deform the part.
Emuge (West Boylston, MA) offers a line of workholding solutions, chiefly for round parts. Emuge workholding design concepts include an array of elements such as sleeves and collets which expand or contract to hold the workpiece for machining or inspection operations.
Their systems can hold ODs of 5 mm with a chuck, or 8-mm IDs with an arbor. The company offers both mechanical and hydraulic solutions, however according to David E. Jones, clamping manager for high-volume work, they stress a mechanical solution due to the wear factors which can occur on hydraulic systems in a high- production environment.
Jones notes that loading clearance—the space between the part and the workholding device—can be a problem with small parts, particularly with high-volume automated operations. System wear can introduce inaccuracies and operation failure during the load/unload cycle; this is where a mechanical system would show advantages as well.
Among the spectrum of workholding products offered by Mitee-Bite Products (Ossipee NH) are a series of low-profile edge clamps for securing extremely small parts. One style that’s very popular is called the Pitbull clamp. The smallest version clamps are as low as 0.075" (1.9 mm) and produce 650 lb (2891 N) of holding force.
When the parts are too small to clamp, Mitee-Bite has developed a wax-based compound (MiteeGrip) that produces 40 psi (275.8 kPa) of holding force, an alternative to double-sided tape. This product is heated and then allowed to cool. Parts are now secure and can be machined using coolant to prevent heat build-up. When the machining operation is complete, reheat to remove parts with ease.
Micromachining for medical applications ranks among the smallest workholding assignments. Matt McCormick is the Prototype Instrument Maker at the Eye Concepts Lab of the Dohney Eye Institute of the medical facilities of USC (Los Angeles). His job is to create prototypes of new instruments surgeons have devised for very delicate eye surgery. According to McCormick, "We use whatever works and that includes Super Glue, Bondo, adhesives from Loc-Tite, and tape from 3M. But among the most commonly used materials are the waxes and tapes supplied by Mitee-Bite."
Instruments made include forceps, scissors, and cutters. A typical assignment would be to machine an instrument 2" (50-mm) long, 0.006" (0.15 mm) in diam with 0.004" (0.10-mm) features. The surgeon will have to maneuver this tool after passing through a 0.5-mm-diam port in the eye.
A typical pass might involve a part secured by double-sided tape, cutting with a 0.005" (0.13-mm) end mill with a small amount of workpiece penetration per tooth per revolution and running on a Haas OM1 machining center.
FixtureWorks (Fraser, MI) offers a number of workholding products, all mechanical workholding products. The smallest holds work down to 1/4" (6.4-mm) diam.
Among the more unique devices for small parts is the OKvise, which has a smooth contact surface specifically for applications requiring the holding device not mar the part surface.
Another is a low profile one-touch clamp that holds small parts by the edge or top through a cam action and is activated by a single handle.
For handling irregular shapes there is a spring-loaded, plunger-type clamp. It’s often used to pre-position a part before final clamping.
Among the specialty workholding devices from Pascal Engineering Inc. (Elk Grove Village, IL) are hydraulically operated hole clamps. When mounted into a fixture plate, the models designated CGH are designed to grip piece parts with holes that are either drilled or cored as commonly found in die castings. The part is fitted over the extended shafts of the hole clamps. When initially activated, the tapered center stem pulls down expanding the gripper collets into the hole. As pressure builds up, the collets will pull the part down against the top surface of the clamp, securing it into the fixture.
The pivot clamp CGP models apply holding or positioning force against a part’s side surface using mechanically leveraged spring pressure. Hydraulic pressure centers the shaft to the straight up position to allow part loading. When the pressure is released, the shaft pivots up to 3º to secure the part against a reaction stop.
Holding Small Parts Magnetically
Magnets are most often selected because they can hold more parts on a single setup and allow clear access to a part’s surfaces without clamps.
With a magnet, the method of holding is fundamentally different. Instead of squeezing across a section, the magnetic attraction basically "sucks" the part to the magnet—delivering clamping force from one face only.
The fundamental limitations on the use of magnetic workholding are: The workpiece must be ferromagnetic and the magnet must generate enough pull to hold the part without movement. Typical electro-permanent milling magnets generate about 11 tons/ft2 of pull force but, for small parts measured in inches, this translates into only 170 psi (756 N). For milling small parts, therefore, ensuring sufficient grip to prevent part movement becomes critical.
With high-speed machining techniques, cutting forces are very low and magnetic clamping becomes a practical solution. Alternatively, parts can be located using pins or keys to prevent movement and the magnet simply holds it in place. And, the magnet can also be used to generate side pull into a corner, an effective way to both locate and prevent movement.
John Powell, president,
Wen Magnetics (Raleigh, NC)
This article was first published in the April 2006 edition of Manufacturing Engineering magazine.